.MKRRIAM: TRIASSIC 1C! 1TII VOSACKI A. 



47 



53 



54 



KlBS. 



In the Triassic ichthyosaurians, as in the typical Irlttli i/ox<tnn(s, rib articu- 

 lations are present from the anterior cervical region along the whole of the 

 vertebral column well out toward the distal region of the caudal series. Ex- 

 cepting the character of the rib articulation in certain regions, the general form 

 of the ribs in the Triassic ichthyosaurs is not materially different from that 

 in the later genera. The shaft is generally considerably flattened, excepting 

 at the distal end, where it may be nearly round in 

 cross-section. In all excepting the most ante- 

 rior and posterior ribs the anterior and poste- 

 rior sides of the shaft are more or less deeply 

 grooved. Compared with Ichthyosaurus, the 

 shaft is possibly somewhat shorter in the middle 

 dorsal region of Cymbospondylus (pi. 7, fig. 1) 

 and somewhat heavier in the dorsal region of 

 some species of Shastasaitms. 



In all of the American Triassic ichthyosaurs, 

 excepting Toretocnemus, the ribs articulate on 

 the vertebral centra and arches, with a single 

 head excepting in the most anterior portion of 

 the column (figs. 53-56) . 



In the best known specimen of Shastasaurus 

 showing the vertebral column (fig. 57) the cen- 

 tra of the anterior cervical vertebrae show both 

 diapophyses and parapophyses. The parapophy- 



ses are situated low down on the centra and are Fi s s - 53 ' 56 - cymbospondyUs 

 somewhat less than half the size of the diapophy- 

 ses. In passing back toward the anterior dorsal 

 region the diapophyses are seen to grow longer, 

 while the parapophyses become much reduced. The distance separating the two 

 apophyses remains nearly the same, or is at least not decreased. On the 

 twelfth centrum of the series the parapophysis is a mere tubercle without 

 an articular face. The last trace is seen on the eighteenth centrum, on which 

 a rudimentary tubercle is present on one side only. Beyond the point of dis- 

 appearance of the parapophyses, the diapophyses continue to increase in length 

 but no division of the rib articulation appears. 



In the older genus Cymbospondylus much the same relations of the rib 

 articulation are seen, and the parapophyses are gradually reduced in passing 

 back through the cervical series until they disappear entirely in the posterior 

 cervical or anterior dorsal region. In one specimen (fig. 58) the parapophyses 



55 



anterior cervical rib; fig. 54, poste- 

 rior cervical rib; fig. 55, anterior dor- 

 sal rib; fig. 56, middle dorsal rib. 



